Hybrid vehicles have an electric drive (EV) mode, in which the hybrid vehicle travels only by power from the motor-generator, and a hybrid drive (HEV) mode, in which the hybrid vehicle can travel by power from both the engine and the motor-generator. Hitherto, various types of hybrid driving devices used in hybrid vehicles have been proposed.
Japanese Patent Application Publication No. 11-082260 shows one type of such a hybrid drive system. The hybrid drive system disclosed therein includes a motor-generator disposed between an engine and a transmission as a result of connecting an output shaft of the engine and an input shaft of the transmission. A first clutch connects the engine to the motor-generator so that they can be separated from each other. A second clutch connects an output shaft of the transmission and the motor-generator so that they can be separated from each other.
When the first clutch is disengaged, and the second clutch is engaged, the hybrid vehicle including the hybrid driving device is put in the electric drive (EV) mode in which the hybrid vehicle travels only by power from the motor-generator. When the first and second clutches are both engaged, the hybrid vehicle including the hybrid driving device is put in the hybrid drive (HEV) mode in which the hybrid vehicle can travel by power from both the engine and the motor-generator.
In the hybrid vehicle, engine output becomes unnecessary when it has become possible to generate a required driving force only by the motor-generator as a result of a reduction in the required driving force. Such a reduction is caused by, for example, a driver taking is his/her foot off a depressed accelerator pedal when the hybrid vehicle is traveling in the HEV mode. Then, the HEV mode is switched to the EV mode. Another situation when HEV mode is switched to EV mode is when battery power for the motor-generator is sufficient, requiring no engine output. On the other hand, when the vehicle is running in EV mode and the required driving force is increased in response to a request for acceleration, such as depressing the accelerator pedal, a shift from EV mode to HEV mode will occur. This requires engine output, because it is impossible to attain the requested driving force only by the motor-generator. The mode shift from EV to HEV will also occur when the charge of the battery for the motor-generator is insufficient, requiring engine output.
According to this known device, it is necessary to perform the mode switching from HEV to EV while disengaging the first clutch and stopping the engine. During the EV-to-HEV mode shift, it is necessary to engage the first clutch to start the engine by using the motor-generator. Further, in some cases, depressing the accelerator pedal causes a request to shift gears of the transmission simultaneously therewith. Further, in some cases, releasing the accelerator pedal causes gear shifting of the transmission simultaneously therewith.